Solar System Log by Andrew Wilson, published 1987 by Jane's Publishing Co. Ltd.

Voyager Project Homepage, http://voyager.jpl.nasa.gov

An alignment of the outer planets that occurs only once in 176 years prompted NASA to plan a grand tour of the outer planets, consisting of dual launches to Jupiter, Saturn, and Pluto in 1976-77 and dual launches to Jupiter, Uranus, and Neptune in 1979. The original scheme was canceled for budgetary reasons, but was replaced by Voyager 1 and 2, which accomplished similar goals at significantly lower cost.

The two Voyager spacecraft were designed to explore Jupiter and Saturn in greater detail than the two Pioneers (Pioneers 10 and 11) that preceded them had been able to do. Each Voyager was equipped with slow-scan color TV to take live television images from the planets, and each carried an extensive suite of instruments to record magnetic, atmospheric, lunar, and other data about the planets. The original design of the spacecraft was based on that of the older Mariners. Power was provided by three plutonium oxide radioisotope thermoelectric generators (RTGs) mounted at the end of a boom.

Although launched about two weeks before Voyager 1, Voyager 2 exited the asteroid belt after its twin and followed it to Jupiter and Saturn. The primary radio receiver failed on 5 April 1978, placing the mission's fate on the backup unit, which has been used ever since. A fault in this backup receiver severely limits its bandwidth, but the mission has been a major success despite this obstacle. All of the experiments on Voyager 2 have produced useful data.

Voyager 2 began transmitting images of Jupiter on 24 April 1979 for time-lapse movies of atmospheric circulation. They showed that the planet's appearance had changed in the four months since Voyager 1's visit. The Great Red Spot had become more uniform, for example.

The spacecraft relayed spectacular photos of the entire Jovian system, including its moons Amalthea, Io, Callisto, Ganymede, and Europa, all of which had also been imaged by Voyager 1, making comparisons possible. Voyager 2's closest encounter with Jupiter was at 22:29 UT on 9 July 1979 at a range of 645,000 km.

Voyager 1's discovery of active volcanoes on Io prompted a 10-hour volcano watch for Voyager 2. Though the second spacecraft approached no closer than a million kilometers to Io, it was clear that the moon's surface had changed and that six of the volcanic plumes observed earlier were still active.

Voyager 2 imaged Europa at a distance of 206,000 km, resolving the streaks seen by Voyager 1 into a collection of cracks in a thick covering of ice. No variety in elevation was observed, prompting one scientist to say that Europa was "as smooth as a billiard ball." An image of Callisto, studied in detail months later, revealed a 14th satellite, now called Adrastea. It is only 30 to 40 km in diameter and orbits close to Jupiter's rings. As Voyager 2 left Jupiter, it took an image that revealed a faint third component to the planet's rings. It is thought that the moons Amalthea and Thebe may contribute some of the material that constitutes the ring.

Following a midcourse correction two hours after its closest approach to Jupiter, Voyager 2 sped to Saturn. Its encounter with the sixth planet began on 22 August 1981, two years after leaving the Jovian system, with imaging of the moon Iapetus.

Once again, Voyager 2 repeated the photographic mission of its predecessor, although it flew 23,000 km closer to Saturn. The closest encounter was at 01:21 UT on 26 August 1981 at a range of 101,000 km. The spacecraft provided more detailed images of the ring spokes and kinks, as well as the F-ring and its shepherding moons. Voyager 2's data suggested that Saturn's A-ring was perhaps only 300 m thick. It also photographed the moons Hyperion, Enceladus, Tethys, and Phoebe.

Using the spacecraft's photopolarimeter (the instrument that had failed on Voyager 1), scientists observed a star called Delta Scorpii through Saturn's rings and measured the flickering level of light over the course of 2 hours, 20 minutes. This provided 100-m resolution, which was 10 times better than was possible with the cameras, and many more ringlets were discovered.

After Voyager 2 fulfilled its primary mission goals with its flybys of Jupiter and Saturn, mission planners set the spacecraft on a 4.5-year journey to Uranus, during which it covered 33 AU (about 5 billion km). The geometry of the Uranus encounter was designed to enable the spacecraft to use a gravity assist to help it reach Neptune. Voyager 2 had only 5.5 hours of close study during its flyby, the first (and so far, only) human-made spacecraft to visit the planet Uranus.

Long-range observations of Uranus began on 4 November 1985. At that distance, the spacecraft's radio signals took approximately 2.5 hours to reach Earth. Light conditions were 400 times less than terrestrial conditions. The closest approach took place at 17:59 UT on 24 January 1986 at a range of 71,000 km.

The spacecraft discovered 10 new moons, two new rings, and a magnetic field (stronger than that of Saturn) tilted at 55 degrees off-axis and off-center, with a magnetic tail twisted into a helix that stretches 10 million km in the direction opposite that of the sun.

Uranus, itself, displayed little detail, but evidence was found of a boiling ocean of water some 800 km below the top cloud surface. The atmosphere was found to be 85 percent hydrogen and 15 percent helium (26 percent helium by mass). Strangely, the average temperature of 60 K (-351.4 degrees Fahrenheit, -213 degrees Celsius) was found to be the same at the sun-facing south pole and at the equator. Wind speeds were as high as 724 km per hour.

Voyager 2 returned spectacular photos of Miranda, Oberon, Ariel, Umbriel, and Titania, the five larger moons of Uranus. In a departure from Greek mythology, four of Uranus' moons are named for Shakespearean characters and one-Umbriel-is named for a sprite in a poem by Alexander Pope. Miranda may be the strangest of these worlds. It is believed to have fragmented at least a dozen times and reassembled in its current confused state.

Following the Uranus encounter, the spacecraft performed a single midcourse correction on 14 February 1986 to set it on a precise course to Neptune. Voyager 2's encounter with Neptune capped a 7-billion-km journey when on 25 August 1989, at 03:56 UT, it flew about 4,950 km over the cloud tops of the giant planet, closer than its flybys of the three previous planets. As with Uranus, it was the first (and so far, only) human-made object to fly by the planet. Its 10 instruments were still in working order at the time.

During the encounter, the spacecraft discovered five new moons and four new rings. The planet itself was found to be more active than previously believed, with winds of 1100 km per hour. Hydrogen was found to be the most common atmospheric element, although the abundant methane gives the planet its blue appearance. Voyager data on Triton, Neptune's largest moon, revealed the coldest known planetary body in the solar system and a nitrogen ice volcano on its surface.

The spacecraft's flyby of Neptune set it on a course below the ecliptic plane that will ultimately take it out of the solar system. After Neptune, NASA formally renamed the entire project (including both Voyager spacecraft) the Voyager Interstellar Mission (VIM).

Approximately 56 million km past the Neptune encounter, Voyager 2's instruments were put into low-power mode to conserve energy. In November 1998, twenty-one years after launch, nonessential instruments were permanently turned off. Six instruments are still operating. Data from at least some of the instruments should be received until at least 2025. Sometime after that date, power levels onboard the spacecraft will be too low to operate even one of its instruments.

As of March 2010, Voyager 2 was about 92 AU (13.7 billion km) from the sun, increasing its distance at a speed of about 3.3 AU (about 494 million km) per year.

Both Voyager spacecraft carry a greeting to any form of life. The message is on a 12-inch gold-plated copper disk (right) containing sounds and images that portray the diversity of life and culture on Earth.

One of the musical selections on the Voyager gold record is Chuck Berry's 1950s hit Johnny B. Goode.

Voyager 2 is escaping the solar system at a speed of about 3.3 A.U. (495,000,000 km) per year.

Voyager 2 is the only spacecraft to visit Uranus and Neptune.

A 30-cm gold-plated copper disc, together with a needle and playing instructions, is mounted on the body casing. On it are recorded natural Earth sounds, 90 min of music, 115 pictures, and greetings in 60 languages.

It is departing our solar system in a different direction than Voyager 1.